1. Field of the Invention:
This invention relates to lens position control devices for use in cameras, video cameras, etc.
2. Description of the Related Art:
A photographic lens having a magnification varying function for a video camera has, in most cases, been constructed as shown in FIG. 6.
In FIG. 6, reference numeral 1 denotes a lens group (F) for focus adjustment; 2 denotes a lens group (V: variator) for varying the image magnification; 3 denotes a lens group (C: compensator) for bringing the sharply focused object to the right place when varying the image magnification; and 4 denotes a lens group (R: relay) for forming an image. Of these, the lens group 2 and the lens group 3, as shown in FIG. 7, have to the shortest focal length (wide-angle end). When zooming is performed, lens groups 2 and 3 move in differential relation between the wide-angle end (W) and the longest focal length position (telephoto end: T).
Meanwhile, when focus adjustment (focusing) is performed, the axial position of lens group 1 is varied by moving it forward as the object distance shortens, as shown in FIG. 8. In this connection, it should be pointed out that, because the "F position" (the position of the lens group 1) and "1/distance" are in proportional relation, in order that the minimum object distance is reduced, the total focusing movement must rapidly be increased, becoming infinite when focusing on an object at a distance of 0 cm.
FIG. 9 shows the variation of the total focusing movement (F position) of the lens group 1 with variation of the focal length of the entire system where the ordinate is the F position and the abscissa is the focal length with the object distance as a parameter taking representative values of .infin., 3 m, 2 m and 1 m. As is evident from FIG. 9 in the conventional lens system, so long as the object distance does not change, even if the focal length is changed by zooming, there is no need to re-adjust the position of the lens group 1. In other words, that lens group which partakes in focus adjustment and that set of lens groups which partakes in focal length adjustment are altogether independent of each other.
FIG. 10 shows particularly that part of the conventional mechanical mounting for this lens which constitutes the interlocking mechanism for both the lens group 2 as the variator and the lens group 3 as the compensator. A holder 5a containing the lens group 2 and another holder 5b containing the lens group 3 are movable as guided by a pair of bars 6 and 7, while their positions are defined by a cam sleeve 8 having camming slots 8a and 8b in cooperation with pins 11 and 12 fitted therein, thus realizing the as-designed accuracy of position control. The zooming operation is realized by rotating a zoom ring 13 which is operatively connected through an interlocking member 14 to the cam sleeve 8 so as to rotate the cam sleeve 8.
Such a conventional type of zoom lens for a video camera has its close-up capability practically limited to 1 m or thereabout as the minimum object distance, as has been described above, and is not suited to, closer focusing.
So, to solve this problem and still to simplify the structure of the operating mechanism, instead of using the lens group 1 (F) in focusing, another lens group may be moved as is known in the art.
FIG. 11 exemplifies that as the other lens group, the relay lens is moved in part. In this example, the first and third lens groups 1 and 15 (R) are fixed, and the second lens group 2 (V) as the variator changes its axial position with focal length adjustment as in FIG. 6. A fourth lens group 16 (RR) constituting part of a relay lens 4 has both roles of compensation and focus adjustment.
FIG. 12 shows the operation of this lens system where the abscissa is the position of the lens group 2 (V) and the ordinate is the position of the lens group 16 (RR). As is understandable by comparing it with FIG. 9, the use of a lens unit as shown in FIG. 11 leads to the necessity of changing the position of the lens group 16 (RR) both when changing the focal length and when the object distance changes. For this reason, in the lens unit shown in FIG. 11, the two movable lens groups are very difficult to control with a mechanical control device which is in such as shown in FIG. 10, or rather nearly impossible. Therefore, this type of lens unit has found little use in actual practice, though it has the advantage of shortening the minimum object distance.
But, in recent years, the technology of automatic focus adjusting devices has advanced, making it possible to detect whether or not an image is formed sharp on the image plane. It also becomes possible to control the position of the lens so as to bring the image into sharp focus.
FIG. 13, FIGS. 14(A) to 14(E) and FIG. 15 show an example of the automatic focus adjusting device.
In FIG. 13, the video camera has a picture 17 in which there is an area 18 for measuring the object distance. Also, an object to be photographed is assumed to have a contrast 19.
Suppose that part of the object which has the aforesaid contrast is imaged as shown in FIG. 14(A), then the device produces a Y signal output shown in FIG. 14(B). FIG. 14(C) represents the differentiated value of the Y signal, FIG. 14(D) its absolute value, and FIG. 14(E) a signal obtained by peak holding, where the height A indicates the degree of focus.
FIG. 15 is a graph showing the variation of the degree of focus A in the ordinate, as focus lens group 1 (F) of FIG. 6 or the lens group 16 (RR) of FIG. 11, varies its axial position in the abscissa. When the degree of focus takes a peak value, a sharp focus is established at the position B.
Thus, even the lens system shown in FIG. 11 has the possibility of realizing a focusing capability when the principle explained in connection with FIG. 15 is put into practice by providing an automatic focus adjusting device 20 and an electric motor 21 for driving the lens group, as shown in FIG. 16.
If the feedback accuracy and quickness of the automatic focus adjusting device 20 shown in FIG. 16 are perfect, the picture will be obtained without any problem at all. However, in reality, the automatic focus adjusting device 20, because of its operating in cycles of distance measurement, has a delay in response, giving rise to the problem that defocus may occur.
In particular, in the case of changing object distances, since even the type of lens shown in FIG. 6 has never been freed from the possibility of setting the lens out of focus depending on the speed of movement of the object, the new type of lens unit may be equivalent to the old one. However, it is during zooming that if the automatic focus adjusting device 20 delays feedback a focusing defect of the sort never seen in the prior art is caused to appear. This constitutes a problem.
So, to solve the above-described problem, a proposal has been made in Japanese Laid-Open Pat. application No. Sho 62-296110, where, based on the position information of the first lens as the variator and the second lens as the compensator which also has the focusing function and the defocusing information, in which direction and at what speed the second lens is to be moved are derived from memory means, so that the movement of the second lens to the compensated position is started in earlier response to movement of the first lens. Another concept of the same kind is proposed in Japanese Laid-Open Pat. application No. Sho 62-284316. The common technique in both proposals is that the position information of at least the variator lens as the first lens is detected as the absolute position information, and still, as another position information, the absolute position information of the compensator-cum-focusing lens as the second lens is detected. Based on these two pieces of position information, data is chosen from the memory means so that the movement of the second lens is controlled.
By the way, in FIG. 12, the locus of the RR lens for an infinitely distant object lies one-sidedly of any other loci. Beyond the locus for .infin., there is an impossible-to-focus zone in which focusing can never be effected to suit any object distance, that is, a so-called inhibiting zone for the RR lens. (After the lens has once entered into this zone, as exceeding the terminal end for infinity of the focusing movement, it is no longer focusable.) When the running RR lens strays into this zone, the image is largely blurred. And it gets harder to quickly correct the focusing direction and bring the image into focus again. If the boundary of this inhibiting zone is a straight line perpendicular to the axis for the movement of the RR lens of FIG. 12, it would be possible to use a mechanical stopper. In fact, however, this inhibiting zone for every position of the variator is distributes like the zone C of FIG. 5, or the hatched zone of FIG. 12. Therefore, the size of the inhibiting zone for the RR lens must be altered as the function of the focal length of the entire system. Up to now, however, the difficult problem of preventing the RR lens from accidentally entering into the inhibiting zone has been left unsolved.